non-gravity on 11 December 2010
Sony will want to launch PS4 close to the Nextbox. By launching Kinect Microsoft ensured they won't launch another console for at least 2-3 years so that makes it end 2012-2013.
I can see Nintendo launching in early 2012- end 2012 at the earliest, but if I were Sony I wouldn't worry about them to much.
Sony should make plans for a launch in 2013
